From the sci-fi movie Stargate and the spin-off television series Stargate SG-1, the Jaffa are humanoid incubators for infant Goa'ulds. They also serve as infantry and bodyguards for the Goa'uld System Lords.
Jaffa are created when Goa'uld technology is used to transform an otherwise normal human, placing an incubation chamber behind an X-shaped incision in the belly of the host. The new Jaffa is then robbed of his/her own immune system and requires the healing powers of a Goa'uld symbiote in order to live.
Once a Goa'uld symbiote is placed in the Jaffa's belly, and until the point at which the symbiote matures and must be removed, the Jaffa benefits from an extended life (100+ years) and semi-perfect health. Thus the Goa'uld created the perfect slave/soldier: each Jaffa needs its Goa'uld symbiote to survive, so they fight together with the System Lords because the eradication of the Goa'uld would mean the eradication of the Jaffa, as well.
The System Lord Apophis created most of his Jaffa army from the people of the planet Chulak, but any human can be a suitable host to an infant Goa'uld after the appropriate transformation has taken place. Colonel Jack O'Neill, of the SG-1 team, was temporarily turned into a Jaffa in episode #113, Hathor.
The Jaffa most SG-1 viewers know best is Teal'c. Originally Apophis's First Prime, Teal'c rebelled against the System Lord in episode #101, Children of the Gods, and joined the SG-1 team to fight against the Goa'uld.
Another well-known Jaffa is Bra'tac, Teal'c's former mentor and a recurring character in the series. Also a traitor to the Goa'uld, Bra'tac first appears in episode 111, Bloodlines, where he attempts to prevent Teal'c's son from being implanted with a larval Goa'uld.
Besides Teal'c and Bra'tac, other Jaffa have joined in the fight against the Goa'uld. These rebel Jaffa have struck a tenuous alliance with the Tauri and the Tok'ra in their battle with the System Lords.
The Jaffa weapon of choice is the Staff Weapon, though they are frequently seen carrying zatnicketels, as well.